60 lines
2.0 KiB
Go
60 lines
2.0 KiB
Go
package screencapture
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import (
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"bytes"
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"fmt"
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"image"
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"image/png"
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)
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// CropPNG decodes the supplied PNG, crops it to rect (in PHYSICAL pixels of
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// the source image — i.e. the same coordinate system the PNG itself uses),
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// and re-encodes the result.
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//
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// Why an extra abstraction: the overlay flow captures the entire screen as
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// one big PNG and lets the user pick a region in the WebView. The selection
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// rectangle the WebView reports is in CSS pixels, so callers must scale by
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// the display's backing factor before invoking CropPNG. Splitting "scale"
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// from "crop" keeps this helper focused and unit-testable.
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func CropPNG(pngBytes []byte, rect image.Rectangle) ([]byte, error) {
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if rect.Dx() <= 0 || rect.Dy() <= 0 {
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return nil, fmt.Errorf("invalid crop rectangle: %v", rect)
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}
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img, err := png.Decode(bytes.NewReader(pngBytes))
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if err != nil {
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return nil, fmt.Errorf("decode source png: %w", err)
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}
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// Clip the requested rectangle to the source bounds — defensive guard
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// against off-by-one errors from CSS-px → physical-px scaling.
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src := img.Bounds()
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clipped := rect.Intersect(src)
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if clipped.Empty() {
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return nil, fmt.Errorf("crop rectangle %v has no overlap with source %v", rect, src)
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}
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// SubImage is documented on most concrete image types; the standard
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// library decodes PNGs into one of those, so this assertion is safe in
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// practice. Fall back to a manual copy otherwise.
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type subImager interface {
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SubImage(r image.Rectangle) image.Image
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}
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var cropped image.Image
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if si, ok := img.(subImager); ok {
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cropped = si.SubImage(clipped)
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} else {
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dst := image.NewRGBA(image.Rect(0, 0, clipped.Dx(), clipped.Dy()))
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for y := 0; y < clipped.Dy(); y++ {
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for x := 0; x < clipped.Dx(); x++ {
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dst.Set(x, y, img.At(clipped.Min.X+x, clipped.Min.Y+y))
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}
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}
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cropped = dst
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}
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var buf bytes.Buffer
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if err := png.Encode(&buf, cropped); err != nil {
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return nil, fmt.Errorf("encode cropped png: %w", err)
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}
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return buf.Bytes(), nil
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}
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